Due to prevailing currents and wind, huge masses of waste dumped in the seas accumulate in the North Pacific gyre.

As most plastics tend to float and take a long time to biodegrade, much of this waste is plastic based and consists of everything from carrier bags and condoms to toothbrushes and discarded fishing nets.

Greenpeace has sent one of its ships, the MY Esperanza, to investigate the full extent of the Pacific problem but has also published a report highlighting the fact that marine debris poses and environmental threat to oceans all over the world, from the Arctic to the tropics.

The report, Plastic Debris in the World's Oceans, suggests that 80% of marine waste comes from land-based sources, either dumped on beaches or washed into the seas from rivers or drains.

The waste poses a problem as sea life either becomes entangled in it or ingests it, mistaking it for prey, which can lead to choking, starvation or poisoning.

Even when the durable materials do break down, the smaller pieces of plastic which do remain then become a threat to smaller ocean animals.

Almost 300 species, including whales, sea birds, turtles, seals and fish, have been recorded as being killed by floating waste.

The bits of debris also provide 'rafts' carrying attached shellfish and other species thousands of miles from their native habitats, potentially creating all the same problems that the introduction of alien species on land.

"During the course of the Defending Our Oceans expedition, we have seen coastlines covered in rubbish, but out at sea the problem becomes even greater - with turtles, albatrosses and many other marine creatures becoming entangled in floating plastic or even choking on it," warned Greenpeace International scientist, Adam Walters, onboard the Esperanza.

"The danger to marine life has been known for decades, but the scale of the problem has not been realised. With plastic consumption rapidly increasing globally, plastic has become ubiquitous in the ocean".

Greenpeace is turning its attention to the vast North Pacific trash vortex because its path carries it near to the recently-designated Hawaiian marine reserve.

"It is ironic that this debris ends up floating past the largest marine reserve in the world," said Greenpeace USA oceans campaigner Buffy Baumann.

"While we applaud the decision to designate the Northwestern Hawaiian Islands as a US national monument, we need international action to create properly protected marine reserves around the world.

"In order to counter all the threats to the oceans - from pollution to overfishing and habitat destruction - the world needs to realize that ocean protection must begin on land."

The Greenpeace report concludes that while agreements such as the International Convention for the Prevention of Pollution from Ships go some way towards addressing the problems of waste discarded by mariners, the real solution requires action on land-based waste management.

Public education programmes and increased efforts on reducing waste are needed to protect marine life, it says, and protected areas such as the Hawaiian reserve announced by the USA should be put in place to cover 40% of the world's oceans.